Deep clone of a data structure in Perl


Function dclone in module Storable can be very helpful.

Sometimes you just need to produce a copy of a data structure, cutting ties with the original. What do I mean?

Let’s define some boilerplate:

#!/usr/bin/env perl
use 5.024;
use warnings;
use experimental qw< postderef signatures >;
no warnings qw< experimental::postderef experimental::signatures >;
use Scalar::Util qw< refaddr >;

my $original = {
    some_array => [ 1, 1, 2, 3, 5, 8 ],
    nested_array => [ ['a'..'l'], ['m'..'z']],
    hash_too => {
        foo => 'bar',
        baz => { hello => 'World!' }

sub compare_references ($msg, $orig, $copy) {
   say $msg;
   my ($oa, $ca) = (refaddr($orig), refaddr($copy));
   say "  orig: <$oa>";
   say "  copy: <$ca>", ($oa == $ca ? ' (same)' : ' different');

In short, we have a complex, nested data structure in $original and we want to copy it so that any change we do to the copy will not reflect in the original.

Function compare_references helps us figure out whether two different variables are pointing to the same references, which would also mean that changes done through one of them would also be seen using the second.

Plain copy

Let’s start simple:

   my $copy = $original;
   compare_references('plain copy', $original, $copy);

Running it:

plain copy
  orig: <93871507775160>
  copy: <93871507775160> (same)

As expected, both $original and $copy point to the same underlying array, so the copy is not independent.

Shallow copy

Let’s move on to copy all the stuff inside $original:

   my $copy = {$original->%*};
   compare_references('shallow copy', $original, $copy);

This is what we get:

shallow copy
  orig: <93871507775160>
  copy: <93871506848512> different

It seems we got what we were aiming for, right? Not so fast, not so fast.

Let’s scratch the surface and see how $original->{some_array} relates to $copy->{some_array}:

   my $copy = {$original->%*};
   compare_references('shallow copy, first level inside',
      $original->{some_array}, $copy->{some_array});

Running gives us:

shallow copy, first level inside
  orig: <93871506603848>
  copy: <93871506603848> (same)

So, again, there are pieces of the two data structures that are common and this would create some interference between the two. This is why we are calling this a shallow copy: after you remove the external layer… there’s no difference inside.

Let’s move on!

Deep copy

At this point, we understand that we have to create a copy at all possible levels of nesting in arrays and hashes. We could code something for this… but the core module Storable helps us with function dclone:

   use Storable 'dclone';
   my $copy = dclone($original);
   compare_references('deep copy', $original, $copy);
   compare_references('deep copy, first level inside',
      $original->{some_array}, $copy->{some_array});
   compare_references('deep copy, second level inside',
      $original->{nested_array}[0], $copy->{nested_array}[0]);

The result is:

deep copy
  orig: <93871507775160>
  copy: <93871506766800> different
deep copy, first level inside
  orig: <93871506603848>
  copy: <93871507775040> different
deep copy, second level inside
  orig: <93871506727416>
  copy: <93871507657192> different

Now we are doing the copy in the right way!


If you need to do a total deep copy of a data structure in Perl, look no further than dclone in Storable. Well… unless you run into some performance issue, of course!

If you want to take a look at the code for this example, you can find it in the local copy.

Stay safe!

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